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What's the difference between radial and bearing when flying VOR?
What is the difference or correct terminology for flying into a bearing (or radial?) when navigating by VOR? What is the difference? What is the correct lingo for VOR communication?

Radial is the word we use to refer to the actual line on the ground emanating or radiating from a VOR or TACAN, or for that matter from any defined geographic point. Think of it as a spoke on a wheel.
Bearing is the direction or ground track (no-wind heading) we would need to turn to in order to point directly at the VOR, TACAN or Point on the ground.
When on any specific radial, you need to point in the reciprocal magnetic direction to point directly at that point.
E.g., if on the 180 radial, you would point north to put the point directly on the nose. The Bearing to the point would be 360.

I don't believe it is incorrect to refer to a bearing from the station. If you are inbound on the 090 radial your bearing is 270 to the station. At station passage when the flag flips from "TO" to "FROM" your bearing is still 270, but it is now a bearing from the station, and you are on the same radial as your bearing. Right? Can't there be a bearing that takes you away from a station?

@Michael, you are confusing the concept of "bearing" with the concept of heading, or ground track. According to Miriam Webster, the most appropriate definition of "Bearing" is "the situation or horizontal direction of one point with respect to another or to the compass." so it is a bit backwards to use it to refer to the bearing of your own aircraft from the navaid. Your heading to the station might be 270, but bearing means from a point or position. I guess you might say the VOR's * bearing* from you is 270, but that just sounds weird....

I don’t think I’m confused, I just think the term bearing is bi-directional.  The Webster definition you quoted even makes that point.  It might be less common usage the other direction, but I don’t think it’s actually wrong.

I'm not even sure what you mean by "bi-directional". As you said, the word Bearing implies the direction from a point to a second point, i.e.  the direction, heading, or track someone or something at the first point would need to point or move to get to that second point. Once you've passed that point, it makes little sense, and is kinda useless to use the word bearing at all. What would you be trying to communicate? The heading that the VOR would need to move in to get to your aircraft?

"...one point with respect to another..." In other words, between two points - in either direction.  And it says nothing about movement of the points.

And no, not "the heading that the VOR would need to move in to get to your aircraft?" That would be silly because the VOR doesn't move.  Instead it's the bearing of the aircraft FROM the station, vs TO the station.  (you know, what the from/to indicator indicates...)

Okay, for an imaginary VOR operator, it might make sense to tell him the "bearing" of some aircraft from his VOR, (and its not silly, perhaps the aircraft is in trouble and he needs to send help), but my point is, apparently, not getting through. To the pilot in the aircraft, his bearing from the VOR is of little interest.

The point is, (and I'm not sure whether you really don't get this), that the concept, the meaning, the intent, of the word "bearing" is to communicate to someone the direction to move in (by heading or track) to locate or get to an  unknown position  FROM a known position.

Indeed, if I'm in trouble and about to perform a forced landing, then I would transmit my Bearing from a known point, to tell the recipient how to locate me. "Reno Approach, N762B, mayday mayday mayday, I'm 230/ 12 miles [from] Table Mtn...."

Let us continue this discussion in chat.


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What is the difference between Glidepath versus Glideslope?
What is the difference between glidepath and glideslope? How would you define each, and the key differences?

Glideslope: describes the systems that generate, receive, and indicate the ground facility radiation pattern.
Glidepath: is the straight, sloped line the aircraft should fly in its descent from where the glide slope intersects the altitude used for approaching the FAF, to the runway touchdown zone.

Source:

ILS - AviationChef

A glideslope is an inclined plane - the glidepath is the line created by the intersection of the glideslope and the plane of the localizer.

So the glideslope is a line in 2D (height vs distance), while the glidepath is a line in 2D (additionally including the heading)?

glideslope is a plane in 3d and a line in 2d (most prominent in the localizer plane), the glidepath is a line in both 3d and 2d

They serve the same purpose for the pilot, the difference is buried deep in the implementation concepts and administration of the systems. I would almost call it a philosophical difference. They have different points of failure, failures and inspections being distributed vs centralized, ease of confirming correctness, that sort of thing.
A Glidepath is a 3D path calculated by RNAV equipment, most commonly a GPSS receiver. It is based on extrapolating the most recent series of coordinates to a predicted position then comparing that to a mathematical model of the intended path. If the receiver has the wrong map-model or math, then the path of that plane will be wrong even if the calibration of the satellite signals are correct.
A Glideslope is an inclined radio signal, it is not used for calculation, it is a physical signal that can be followed. The equipment indicates where the plane actually is at that moment relative to the signal without extrapolating any past positions. The receiver is not following a map-model, if the signal is not aligned then the slope that all planes follow will not be aligned even if the receiving instruments are correct.

You should add that this definition is only used by some manufacturers (e.g. Boeing). Airbus uses the term F-G/S (FMS glideslope) instead of G/P (glidepath), see e.g. this answer. The other answers refer specifically to ILS.

@Bianfable my answer is independent of manufacturer software UI quirks. I am using the FAA concepts of glideslope(a physical signal on an inclined plane indicating a particular slope; either visual or radio) and glidepath(a charted/calculated or actual decent angle to be taken by an aircraft), as used internally in the agency for terminal procedure development and inspection. Though I may revise my answer tomorrow for clarity, I'm a bit fatigued.

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What is defined as "day VFR" under EASA rules?
I received a document where it it specified that the flight relevant for such document can only take place in "Day VFR" conditions undear EASA regulations.
Is this connected to VMC? What does EASA defines as "Day VFR"?

It's by opposition to Night VFR (NVFR) which allows to fly by night in visual meteorological conditions. So the answer will focus on NVFR, rather than on day VFR.
EASA harmonization over EU countries is done by enforcing Standardised European Rules of the Air (SERA). SERA states that a night flight must be conducted in IFR, and therefore the crew must hold an Instrument Rating.
Prior to SERA, some countries did not allow VFR at night for security reasons (aircraft not visually identifiable, and no transponder), but other did, that's the case for the UK that I'll use as an example here.
When implementing SERA, EU countries can use acceptable means of compliance, as described in ED Decision 2013/013/R and its annex.

NVFR is implemented (or denied) in EU country by country. So rules may differ between countries. UK example:

UK license can be extended for NVFR.
More for UK NVFR: Safety Notice: SN-2012/007.
In practice, VFR at night is permitted in accordance with SERA requirements. Aircraft leaving the vicinity of an aerodrome must maintain 2-way communication with ATC and file a flight plan. The process of 'booking out' is still accepted as an alternative method to filing a formal paper or electronic flight plan in certain circumstances. Similarly, abbreviated flight plans filed in flight are still permitted. There are also more restricting weather minima:

Minimum cloud ceiling of 1500 ft AMSL
Flight visibility of 5 km, or 3 km in the case of a helicopter flying outside controlled airspace
Maintain sight of the surface when flying at 3,000 ft AMSL or below
Minimum height of 1000 ft (or 2000 ft if over high terrain) above the highest fixed obstacle within 8 km of the aircraft except when taking off or landing

Night is defined for  this purpose:

e.g. for UK in Air Navigation Order 2009 Art. 255:

'Night' means the time from half an hour after sunset until half an
  hour before sunrise [...] at surface level.

Aircraft must be compliant for NVFR, e.g. this equipment is required:

artificial horizon
Instrument Lighting
Landing Light
Position and Anti Collision Lights

In countries where NVFR is not allowed, IFR applies at night.

That sounds like a much more practically useful definition of 'night' than the one we have in Sweden (and I'm not commenting on the half hour specifically; that could be any amount of time). I'm not really up to digging out the exact definition right now, but it goes something like "highest point on the sun's disk lower than six degrees below the horizon" or some such. Perfectly scientifically valid, but almost impossible to apply in practice!

@MichaelKjörling: Ah yes indeed :-) Seems something more practical for ships using a sextant.

Would even having a sextant help, though? Because when the highest point on the sun's disk is below the horizon, there would be nothing to aim the sextant towards...

@MichaelKjörling: Oops, read "above" instead of "below". Then your comment is twice right :-)

Ah, yes, then your comment makes more sense. Maybe I should have written "more than" instead of "lower than" in my initial comment, but the grace period for editing that comment has long since passed now and I'm not 100% sure what I wrote is the exact definition anyway (though it's close), so... Maybe I'll dig out the actual definition later. (The exact definition is unlikely to be very relevant to me, since for the time being I'm only aiming for VFR in a VFR-only aircraft type.)

VFR @ night in Germany requires a HG and DG, VSI, - a standard IFR panel plus a VOR receiver, lights and License. The funny description for the beginning of night applies here too. The German Weather Service makes these times public for selected airports. Choose the one closest to yours. See this link (in German).
Else a flightplan is required and communication to radar.
IMHO all overly too many requirements. VFR @ night is not so much more demanding as VFR/day on a hazy day. Considering it IFR because of the "lack of a horizon" shows that these rules are made by folks who never flew at night, or even never flew.

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Missile Impact Velocity
For the past 10 years or so I hear more and more about so called "Hypersonic Missiles" that can hit their targets at speeds exceeding Mach 10.
I wonder about the accuracy about such descriptions. There's no problem with flying at crazy high speeds outside of the atmosphere - but surely, doing so in the lower layers will cause enormous heat and melt down the vehicle.
For example, the SR-71 had to be built from special materials to allow it to fly at Mach 3.2 and even then only at very high altitudes.
Has our understanding of aerodynamics and material science changed so much since the times of the SR-71 ? Or the numbers are simply being thrown around by people who don't know any better ?

Heat is not the only challenge. Total or stagnation pressure increases faster with Mach than temperature. For example, at Mach 10 the ratio of total temperature to ambient is 21 whereas same for pressure is 42,439.  Or, another way to think of it is sea level qbar at M=10  is >1,000 psi, ~100x higher at M=10 than M=1. Drag increases dramatically in terminal phase.

I mean, at mach 10, you can hit a target 500 miles away in under 4 minutes, so being able to soak the heat for that long is a very different question from the SR-71 that wants to last for hours and return to base afterward.

You are correct about the heat problem. It is a major issue with hypersonic air-breathing vehicles operating within the atmosphere or spacecraft returning to Earth at orbital velocities. The choice of materials to construct a vehicle with is largely based on the heating it experiences in certain areas of the airframe. In general, the vehicle will get extremely hot on the nose and leading edges of the wings and control surfaces. Other areas are not as heat critical, so less exotic solutions will be require required in these locations.
Another issue is time, i.e. the length of time that the vehicle experiences hypersonic flight and the rate at which the air frame becomes heat soaked. Simple hypersonic missiles which maintain these kinds of speeds and temperatures for short periods of flight can make do with relatively simple thermal protection systems, whereas an aircraft or a spacecraft intended to spend a great deal of time at these speeds and having the air frame thoroughly heat soaked requires more exotic and expensive solutions.
To date these issues have been addressed with a number of different solutions in creating a thermal protection system (TPS):

Exotic metallic and composite materials - Build the exterior of your craft out of exotic heat resistant materials. The SR-71 Blackbird which you cited above was constructed largely out of titanium for this reason. The MiG-25 opted for a relatively economical, albeit heavy, solution of manufacturing the airframe out of stainless steel. The X-15 research aircraft which achieved speeds of Mach 6 was constructed largely of Inconel. The X-43 hypersonic research aircraft used a leading edge made from carbon-carbon composite.

Ceramic based TPS - This has been a very popular solution for exo-atmospheric vehicles entering the atmosphere at orbital or near-orbital velocities. First pioneered on the space shuttle, it consists of affixing a mosaic of ceramic tiles to the outside of the craft, generally on the bottom surface and other heat critical areas, to provide excellent thermal insulation and heat protection during reentry.  A chief difficulty with these designs has been preventing tiles from being shaken loose with the stresses encountered during launch. This has been a problem for even modern applications like SpaceX’s Starship.

Ablative TPS Materials - These are materials designed to burn off and dissipate the heat encountered during hypersonic flight for brief periods. The X-15 program use the solution of a spray-on cork slurry covering the exterior of the aircraft, which provided limited thermal protection during hypersonic flight.

Boundary layer cooling - common in modern jet engines, the technique requires drilling a mosaic of fine holes in the surface of a part subject to high temperatures, and passing cool air through these, effectively creating a colder boundary layer than the high temperature air in free stream, thus protecting the part.

Vehicle Shaping - One of the best ways to reduce structural heating is to design a vehicle that transfers most of the frictional heating into the atmosphere as opposed to the airframe.  It was found through experiments in the 1950s that blunt shapes work far better than sleek or needle like shapes in order to do that. This is why most hypersonic aircraft or lifting body spacecraft employee a wedge-shaped fuselage which has the additional benefit of serving is a lifting body for either glide or powered flight in the atmosphere when re-entry is complete.  Other examples are cone shaped space capsule designs with heat shields on the base of the capsule.  This was a default with the Mercury, Gemini, and Apollo programs.  Most recently, Boeing’s StarLiner program and SpaceX’s Dragon space capsules make use of this re-entry philosophy for thermal protection.

i would imagine that single use would free up some design considerations that would be concerned with durability.

So an impact velocity of Mach 10 is attainable ?

@shaiko - a look at the Sprint Missile may be informative of what is physically possible, and what size it ends up being. https://en.wikipedia.org/wiki/Sprint_(missile).

There's also perspiration cooling, where you force coolant out through holes in the skin and the boiling coolant is swept away by the airstream to get rid of the heat.

Early reentry bodies used a large copper tip as a heat sink.

@ Peter Kämpf what vehicles use that?


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Where can I find prof. Wittenberg's TU Delft aircraft propulsion notes?
In threads like this one (Why do jet engines have a maximum speed?) I stumbled upon some pics that were taken from prof. Wittenberg's lecture notes at TU Delft. I have looked everywhere on the internet but could not find them. Can someone tell me where I can get them?
Thank you very much
Federico

Have you tried your local library? They are magicians in finding literature from all kinds of places.

Um - yeah the handout in question, VTH-D 14, dates from quite a while before the start of the internet. And it is in Dutch.
I have a treasured paper copy (with an imprint of a cup of coffee), which was already over a decade old when we were lectured on it.

There is an on-line repository of TU Delft publications, entering H Wittenberg in the search area yields quite a few results.


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Wind turbine airfoil on low Reynolds number aircraft
I read that the criteria considered when designing a wind turbine airfoil are slightly different to the ones when designing an airfoil for an aircraft. I was wondering, for a low Reynolds number aircraft, is it okay if an airfoil designed for wind turbines were used, and wondered if there were going to be side effects from doing so.

Wind turbine airfoils do actually work at relative low Reynold numbers. But also other requirements are important in their choice, like being aerodynamically insensitive to dirt, unsteady characteristics, stall characteristics, thick enough to be structurally strong, ... Do you have a particular design or aircraft in mind?

A 200 foot wind turbine blade actually will have a much higher Reynolds number near the tips, and a more wing-like relative wind.  Since turbine blade (and propeller) speed varies depending on distance from the center, you might notice differences in the airfoil cross section.  Why not study aircraft airfoils.

@sophit, I am trying to design a glider with as little drag as possible. I am looking into other airfoils from Drela and Eppler, but wanted to know if it was okay to consider wind turbine airfoils.

Understand, nice project. Well, as said wind turbine airfoils (or, more generally, rotating wing airfoils) must obey other type of requirements than fixed wing airfoils. Furthermore most wind turbines have variable pitch blades and therefore their blades operate always around their best AoA... I wouldn't suggest you to have a look at these airfoils. Anyway NASA LS airfoils are the ones normally used for wind turbine application, especially because their aerodynamic characteristics are stable in respect to dirt on the leading edge. Hope it helps.

@sophit Thank you for the input!

A little searching found the naca 4415 at the end of a turbine blade.  The 4412 might be worth a look too.

Anyway there's a lot of material here about glider airfoils. Maybe you can post a new specific question if you have some doubt: there's a lot of glider enthusiasts around here :-)

Do not take the origin of an airfoil as a criterium for its selection. Judge it purely on its merits for the intended purpose. If a wind turbine airfoil gives you better L/D than any other, use it.
For gliders you need to cover several design points with somewhat conflicting criteria and select the best compromise.

Thermalling: The airfoil should allow for a small sink speed at low flight speed. Low structural mass and a high lift coefficient are the most desirable traits here.
Gliding: Here the absolute drag counts above all else. If your maximum lift coefficient allows for a smaller wing (you should size the wing for a desired minimum speed), a slightly higher drag coefficient at low lift coefficients can still result in a better overall optimum.
Also pick a planform with moderate taper and select different airfoils for root and tip. Tip airfoils must stall gradually and allow some margin for the aileron.

It is important what you design the glider for: A foot-launched, foldable design might need a very different airfoil from one optimized for ridge lift or even aerobatics.


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Is there a way to access Flight Data Recorder information for major incidents?
I am working on a school project where I am hoping to use by-second data from flight 1549 (miracle on the hudson). Is there a way/place to acquire flight data recorder information for these major accidents? NTSB seems to be offline and has been for quite a while. Thanks


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Holding entry is counted as a one turn holding?
If I ask ATC 'request direct to ABCDE waypoint and one turn holding', do they expect me to do the teardrop entry or parallel entry if I need to and fly the whole holding pattern after that?
Or teardrop or parallel are counted as one turn holding?
I looked up the textbook and handbook, but could not find it anywhere...
If you could back up your opinion, that would be very appreciated.
Thank you in advance.

Welcome to aviation.SE! This question might be helpful too.

Personally I have no expectations whatsoever. Sometimes the student (or instructor) requests multiple holds and they end up only doing one, and sometimes it's the opposite. My general technique is to say "Hold over ABCDE as published and advise ready to exit the hold," which lets you tell me when you are done doing whatever you need to be doing.
If absolutely pressed, I would say that the entry to the hold—however you do it—does not count for the "one turn holding;" the "one turn" starts once you are over the holding fix and aligned in the proper direction.
Source: Students request holds every so often in my airspace. It doesn't happen daily but it does happen.

The first time you pass over the holding fix is the entry. Each additional time you pass over the holding fix completes one turn.
The first turn is often not a complete racetrack, e.g. a teardrop or parallel entry, but it still counts as a turn. Most importantly, such a turn qualifies as a hold for maintaining currency or for checkrides (per my DPE). You do not need to fly even one complete racetrack unless you did a direct entry; a course reversal is enough.
That said, I never requested “one turn” during training. I would request to hold as published at an IAF, and unless there was traffic (typically another student doing the same thing) ATC would simply tell me to advise when I was inbound. I could do as many or as few turns as my CFII desired.


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Does the Remote Indicating Compass (old type) both have precession motor and a torque motor?
I am teaching aviation students theory for the ATPL Exam. And one of the subjects, instrumentation, there is question on the exam that has stumped me.
It claims that the torque motor that will drive the outer gimbal to the correct heading if it gets an error signal, is also used as the system that will erect the gyro and keep it from toppling?
But in all, but limited, information I can find, we are talking about a precession motor. That does the primary job, correcting the error.
Which begs the question: Are there two motors? A precession motor and a torque motor that is activated with a mercury tilt switch if the gyroscope starts to topple?

I sense another ASE question brewing here-- "In the context of the internal workings of aircraft instruments, what is the difference between a 'precession motor' and a 'torque motor'?"

A torque motor is a type of rotary actuator.  Usually there is an output arm that moves like a clock hand, or just an output shaft you attach something to, and there is a resting position, and when you apply an increasing DC voltage, the arm moves off the resting position in proportion to the voltage increase.   Take away the voltage and it goes back to rest.

It depends on the age of the instrument.  The signal amplifier passes a DC current to the ‘precession motor’ to precess the gyro in azimuth back to the correct heading. Later versions use a torque motor instead.  This should not be confused with the separate erection system torque motor that keeps the gyro spin axis horizontally aligned. I’m an ATPL TKI and that’s the info that I have.

Thanx Nic,
much appreciated answer.


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What is the difference between a tear strap and a doubler?
Is there a difference between a tear strap and a doubler? If I get it right, the tear strap reinforce the main structure locally and prevent the propagation of tears, while the doubler, well, it has the same function. Is the difference lying in their geometry?

The difference is the doubler has a broader application, as a general reinforcement of a skin, web, or flange to increase tensile strength, compression strength or stiffness by making the part thicker over an area, generally with multiple load paths to absorb.
A tear strap, being a "strap", is more targeted as a tension-only reinforcement that is applied along a particular load axis to offload some of the tension load on the underlying part along that axis.

The narrower section of the strap is able to reinforce a targeted path, it makes sense.
Just one question on the other part of the answer:
If the strap increases the tensile strength, shouldn't it be able to improve compression strength and stiffness as well? Or its installation procedure it's not the same as the one used to install a doubler, so somehow it influences the final function of the strap?

Somewhat, but If it was intended to improve compression strength as well, it will have a cross section more optimized for stiffness and buckling resistance, like a hat section or U channel or something.  A strap being a thin (relatively) strip of metal will contribute little resistance to buckling of a panel for a given amount of material and would be a very inefficient way to add compression strength, pound for pound.


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Cessna 172 - electrical system - checklist confusion
I have been studying about Cessna 172 electrical systems and some checklists.
In the checklist, it says - AVIONIC BUS 01 and 02- TURNED OFF - BEFORE ENGINE START.
And the reason could be that in order to avoid transient voltages entering the sensitive avionics.
I found the schematic of electrical system here

In the schematic, it shows Alternator circuit is directly connected to ESS BUS via ELEC01 and ELEC02 without any control switch. And the ESS BUS in turn directly connects to avionics(PFD).If the goal is to protect avionics, then having a connection between alternator and ESS BUS wouldn’t be a problem?

Good question! The POH says "Placing the switch in the ARM position during the engine start cycle allows the standby battery to help regulate and filter essential bus voltage during the start cycle," but it's not clear to me why this would apply to the ESS BUS but not the main buses. Perhaps the diodes connecting the ESS BUS to the main buses are designed to absorb transients?

As per my understanding, diodes are devices that allow electricity to flow in one direction(only), like a valve and they do not really ‘absorb’ any transients.

It is connected via Alternator Relay, not directly connected.

In my opinion this disconnection of the "sensitive avionics" when starting/stopping the alternator is no more relevant. It seems to be the persistence of a precaution, once useful, "just in case" some equipment were not properly designed.
As you observed, the PFD and other equipment on the ESS bus are fed by the alternator even when switching it On/Off, hence are subject to transients like equipments connected to AVN 1/2 would be if the buses where not disconnected. So these ESS bus equipments are not considered as "sensitive avionics" anyway, and this could be extended to the ones on AVN 1/2 as well, in modern times.
My best guess is not powering the AVN 1/2 when starting the engine is a mean to give the starter all the available power in case the battery is not fully charged.
Voltage transients are always present
The alternator generates AC which is rectified and filtered or regulated to produce acceptable DC. This DC source then feeds various loads, which have some reactance (capacitance or inductance) as no actual load is purely resistive. Two kinds of transients are observed:

When the alternator and starter coils are switched on or off, a higher voltage appears at their terminals, due to the back-emf created by the self-inductance. This back-emf was once used in the automotive distributor ("Delco") to generate the high-voltage required by the spark plugs from a low DC voltage.

When a reactive load is connected or disconnected to/from a DC source, transient states are observed, where the voltage is not the one of the DC source.

In both cases positive or negative high-voltages can be created. Avionics equipment of the past were sensitive to these transients, but that's not the case of modern avionics. Modern avionics uses modern ICs and is protected against such known transients.
DC transients are AC
The time DC stabilizes, the voltage varies, between a low and a high value, this may occur once or several times. During this period the current is actually AC. In DC the parasitic reactance of each load (capacitance and/or inductance) plays no role, however this is not the case in AC. E.g. a capacitor, which is made of two unconnected plates separated by a dielectric, is indeed an open circuit in DC, but starts conducting in AC, the time it is fully charged.
DC equipment subject to AC transients must be designed to work in AC for a small period of time. Modern equipment deals with that by filtering its power source again, and by protecting its signal inputs.
Modern avionics is protected against transients
In modern equipment the input voltage is filtered again, there are clipping diodes (e.g. reverse Zener diodes) to limit the peaks. This comes at the price of harmonics creation which must be dealt with too. Thus the IC has calibrated + and - voltage references. Then each input is prevented to move outside the reference range using direct diodes.
You can read more in this article about the protection of a modern microcontroller. From this article, the difference between an unprotected input (top) and a protected input (bottom):

The protection is both internal to the IC (diodes) and external (resistor and capacitor).
Thus with old equipment it made sense to start the alternator with the avionics disconnected, the time for the transients to disappear. For modern avionics it is likely unnecessary, but the precaution is still there.

Completing some comments.

The diodes linking buses are to isolate them, e.g. ESS is isolated from ELEC 1/2, to prevent ESS fed by the standby battery to also feed ELEC 1/2, while at the same time ELEC 1/2 can feed ESS.

The diode in parallel with the test push-button allows to charge the standby battery with current from ELEC 1/2, but prevent the standby battery to feed them.

These diodes are not aimed for transient filtering.

Just to know, how these transients from alternator were handled in the old times?

@Theja: The usual method has been to use a big capacitor on the alternator and delay the supply of avionics until DC is stabilized. Current avionics uses CMOS (high impedance) instead of TTL (low impedance) ICs. High impedance means susceptibility to electrostatic discharges (a given voltage creates a higher current in a higher impedance), so CMOS ICs are better protected (fast clamp diodes). This protection in turn makes them accepting some supply transients.

As someone who spent 25 years in avionics design, I'll agree that modern avionics include input protection. But I've seen that, especially with general aviation, aircraft power is garbage. Newer aircraft designs are better, but most of the fleet does a really bad job. In tests, we measured transients 2 to 3 times the industry standards. And I've seen failed units where the entire protection circuit was smoked. Turning off the avionics master switch is cheap insurance.

@Gerry: I fully trust your experience and won't argue, that's a good addition. However I note in the present case, the same kind of avionics is connected to ESS and AVN buses, e.g. PFD, ADC/AHRS and NAV 1 are both connected to ESS and AVN 1. So if they are "sensitive", they are not protected when connected to ESS. Same for radios with AVN 2, only XPNDR is only connected to an AVN bus, and is really protected. This is a mystery.

Welcome to the community.
You are correct in thinking that we are protecting sensitive avionics during an engine start.
Both the main battery, external power and alternator provide electrical power to the ELEC BUS 1 and ELEC BUS 2.  The alternator switch should not be on, though during an engine start.
These two buses in turn provide electrical power to the ESS BUS and the respective AVN BUS.
As you mentioned, the avionics buses are depowered by turning off the AVIONICS BUS 1 and AVIONICS BUS 2 switch during an engine start.
The ESS BUS is powered by both the main battery and external power as well as the STANDBY BATTERY.  On the bottom of the standby battery box on the right side, it says MAIN BUS VOLTAGE SENSE and I will make the assumption the standby battery will kick in to power the ESS BUS if main battery voltage drops too low during an engine start.  In this way, the ESS BUS is protected from transient voltage fluctuations that cause damage to the avionics.  There isn't a possibility of an overvoltage with the alternator offline.

What would happen if i turn on alternator and it produce over voltage ?

How would an over voltage effect the ESS voltage ?

If an overvoltage occurs, you can begin to cause electrical issues up to frying the electrical components of the system.  You will see that in increase in the voltage on the PFD or MFD screen.  Follow the POH checklist procedure if that occurs.

The essential bus is connected directly to the standby battery when the Standby Battery Switch is in the ARM position, as it should be for engine start:

Placing the switch in the ARM position during the engine start cycle allows the standby battery to help regulate and filter essential bus voltage during the start cycle.

A battery is a device meant to hold something at a constant voltage. Thus, as the quote implies, having one in direct contact with the bus helps filter any transients from the bus.
During startup, the voltages on the main buses are lower due to the very large currents being provided by the battery, while the voltage on the ESS BUS (provided by the standby battery) will not be dropped. Those diodes only allow current to flow when the voltage on the left side is greater than the right side. So the diodes connecting ELEC BUS 1&2 to the ESS BUS should be inactive. Particularly large positive transients may still cause some current flow through the diodes, but the transients should be clipped compared to ELEC BUS 1&2 where there is no such protection.
In short, the ESS BUS is essentially disconnected from ELEC BUS 1&2 during startup, as long as the standby battery voltage is greater than or equal to the battery voltage while the starter is cranking.

The diode itself would not clip any transients I guess. Could there be any additional circuit in the system that take care of this ?
( not shown in the attached in the figure)

@Theja The diode will clip transients. Suppose the ELEC BUS 1&2 are at about 20 volts and the ESS BUS is at 24 volts (just example numbers). A 4 volt transient on the ELEC BUSes is just not enough to get current to flow through the diode so the circuit on the right won't see it at all.

What about the positive transients?
The diode will conduct when ELEC BUS VOL > ESS BUS VOL?

@Theja A positive transient of, say, 5 volts will get through the diode to some degree, but it will still be clipped- the right hand side will only see a pulse of 1 volt. The diode conducts when ELEC BUS > ESS BUS, but by the very nature of operating the starter this should not normally be the case.

Clip is probably the wrong word to use. The diode won't clip transients, but attenuate them, taking a couple of volts (depending on the diode) off the transient, passing any remaining voltage increase on.

As shown on the diagram, the [ALT Output] is never "directly connected" to [ELEC BUS 1].
It is provisionally connected through [ALTERNATOR RELAY].  When the Alternator Relay is open, [ELEC BUS 1] is only connected to the battery.
[ALT MASTER] controls both ACU [Power In] and [ALTERNATOR RELAY]. [Power In] provides the field current for the Alternator.
You do not wish to provide field current to the alternator while starting, for two reasons:
(1) That would be stupid. You're going to use electricity to turn the starter motor, to turn the alternator, to provide electricity.  Doing that just throws away power: you get less than you put in.
(2) The field current is unlimited* when the alternator is stalled. You can flatten your battery and burn out your alternator. You don't want to provide field current unless your alternator is running at speed.
(*Controlled by the ACU. You should consider posting any further questions about circuit diagrams, or alternator field currents, to Electrical Engineering stack exchange, rather than avionics.)


text:
Approach beacons location
What is the characteristic of the 2x "approach beacons" mention in the Singapore's AIP Runway 02L. (Picture attached below ). I cant find anything in the ICAO Annex 14 that describe the location of this beacon to be placed with the CAT II Approaching Light System on a runway.

A search for "approach beacon" in ICAO and CASA of Singapore online documentation doesn't yield any result, except CASA AIP entries like the one you show in the question. So it's indeed a mystery. I'm proposing three possible explanations.
1: "Approach beacons" refers to airport beacons
There are indeed two airport identification beacons at WSSS:

Source: Jepessen, via virtualairlines.eu
2: Additional lights in the approach path
These two black squares on the right of the ALSF-2 drawing:

Source: Jepessen, via virtualairlines.eu
I don't know what they represent, and wasn't able to find a an explanation in the Jepessen legend. ICAO prohibits a light at this position when the ALS is active. From ICAO Annex 14:

5.3.4.29: b) no light other than a light located within the central part of a crossbar or a centre line barrette (not their extremities) shall be screened from an approaching aircraft.

But the AIP doesn't state these beacons are part of the standard precision approach. These two beacons must be external to the ALSF-2 standard, and wouldn't be used when ALSF-2 is operational.
3: Additional lights in the approach path
The lightning system is an ALSF-2, implementing ICAO standard for the lightning  of a Cat II/III precision approach:

Source: ICAO Annex 14, via Switzerland FOCA
But on an aerial view, there is an additional mast with two lights in the stopway, on the centerline:

Source Google Maps
In some drawings for ALSF-2 there are two beacons at about 50m (150ft) from the threshold, between the centerline and the side row lights, never described. This one is from FAA:

Source: FAA
While the position isn't exactly the same, this additional mast can be for the additional lights, again not used when the ALSF-2 is active.

+1 for a solid research effort despite not giving a definitive answer


text:
Can we log flight time or PIC time together?
My friend rented a plane, and we ended up flying together while teaching each other. Both my friend and I are CFIs; can we log flight hours or PIC time together?

Hi. I'm guessing this is a speculative scenario, as I certainly hope that if this actually happened, at least one of the two CFIs would know the answer?

Although we shouldn't ever stop learning, I think it would be a stretch to call two CFIs flying together "teaching" under part 61 if what you are really trying to do is build time while chatting about personal techniques...

It's allowed, provided the pilot not flying is providing instruction to the pilot flying and the pilot flying is sole manipulator of the controls. The sole manipulator can log PIC time under 61.51(e)(1)(i) and the pilot providing instruction can log PIC time under 61.51(e)(3). The training needs to be properly logged in accordance with 61.51(h) and 61.189.

Can the training be ad hoc, or does it have to follow a predetermined plan?


text:
Do hot air balloons have flight data recorders and cockpit voice recorders?
I'm wondering if hot air balloons also have flight data recorders and cockpit voice recorders like commercial jet aircraft. I mean, if there isn't one and an accident happens, how are investigators supposed to know what happened? Also, if there is one, where is it stored? I know that FDRs and CVRs are usually stored in the tail on commercial aircraft, but a hot air balloon does not have a tail, so where are they stored? Does anyone know anything about this? I would like to know. Thanks in advance!

Would the downvoter please add some feedback as to why this is a bad question?

Yeah, downvoters, please add some suggestions for improvement or other advice so I can improve next time.

Flight recorders aren't very common outside of airliners and military aircraft, since their installation is fairly complex. Per balloon regulations, they don't require certified CVR or FDR.
Hot air balloons don't have much in the way of instruments, traditionally just an altimeter and a variometer. Today, they usually use a handheld-style GPS unit, which has its own recording capability, but isn't hardened like commercial FDR. There's no need for a CVR, as balloons only need a single operator, so there's no crew communications to record. They are passively safe (most failures lead to a slow descent), and any accident's cause is normally quite obvious.
It's like why horses aren't fitted with dashboards. Perhaps you could strap one on, but should you? That said, you can use a handheld device to navigate, and that's what balloon operators do.

Audio recording could be useful to provide non-invasive data on burner firing. It would also record radio and voice communication with ground crew, which is critical to safe takeoff and landing (e.g. avoiding power lines).

@user71659 Perhaps. But burner failure is a non-issue, it just results in a forced landing. Comms with ground crew might matter, but it's way easier to install a dictaphone app on a crew's phone (or request the recording from the carrier, phone calls aren't really private) than a FAA-certified black box on the balloon.

Crew's phone? Hot air balloons don't rely on cell phones for important reasons: poor coverage at altitude due to overlapping cell site visibility and potentially landing somewhere without coverage. They're supposed to use airband radio, many use services like commercial 2-way FM and FRS instead.

@user71659 Yeah, they do occasionally use a CB walkie-talkie. The safety culture in there is slightly below zero.


text:
Had there ever been a self-ejecting FDR/CDR?
Ever since the invention of Flight Data Recorders (FDR) and Cockpit Voice Recorders (CVR), they played a crucial role in helping investigators determine the cause of accidents. Since they are so crucial, both are designed to withstand tremendous G forces and high temperature (to protect against fires). However, the nature of some crashes are impossible to withstand, or the crash site made the discovery of FDRs and CVRs virtually impossible (for example see South African Airways Flight 295, United Airlines Flight 389, BOAC Flight 911, etc., there's actually a lot of them, see list of crashes where FDR is never recovered). There are also cases where the aircraft disappeared, or no or only partial wreckage were ever found, not including FDR or CVR (these cases usually happens over oceans, or occasionally over jungles/forests, places where locating the wreckage would be an almost impossible task; see Dominicana de Aviación Flight 603 and the famous Malaysia Airlines Flight 370). Thus, my question is, why didn't they design a FDR and CVR that ejects itself from the aircraft using ejection seat mechanisms prior to an imminent crash? Since data recorders are so important to investigations, they should be guaranteed to be found and not damaged. Although most of the time recorders survived the crash and were found, there's always the possibility of an unsurvivable crash or undiscoverable crash site. Consequently, ensuring the safety of recorders are extremely important. Are there any practical reasons for why such a device wasn't designed and used? I appreciate any help. Thanks in advance!

For civilian aircraft they are under development, see https://skybrary.aero/articles/automatic-deployable-flight-recorder-adfr

In the late 70s when I was working on DHC-5 Buffalos, some of them had an option for an ejectable ELT. It sat on top of the fuselage and could be remotely released. It was a bright orange rounded square shaped lifting body glider that would descend in a circle when released.

United 389 was a CFIT crash; it's not clear to me what sensor data you could use to trigger the auto-ejection in a case like that.

How do you tell the difference between an impending crash and an impending landing?

Surely it's easier to find an FDR if it is still part of the main wreckage, in the known location within what's left of the aircraft, rather than a small item somewhere a couple of miles away in impenetrable jungle? Look at the case of that bloke that went missing in Tenerife - a much larger item to find, and it took about two weeks to find him, and that was with loads of people looking.

@aircraft enthusiast 007, do any of the answers satisfy your question? You should accept one.

Any reasons, downvoter? Please state where I can improve my post to make it useful for future viewers. This question got some very great answers, and I would like my question to be clear and understandable as well. Please inform me of any suggestions in the comments. Thanks in advance!

Civil aviation
This wasn't used since recently but after two tragedies, AF447 in 2009 and MH370 in 2014, it was clear the current equipment wasn't totally adequate, for the former it was difficult and costly to determine the crash site and recover the recorders using ELT or ULB signals, for the latter accident it wasn't possible.
ICAO recommended to extend the mandatory recording time and the homing beacon battery lifetime, and to use ejectable recorders for flights beyond radar coverage. More in: ICAO Global Aeronautical Distress & Safety System (GADSS) and Annex 6 Part I.
As of today:

The homing beacon lifetime has been extended from 30 days to 90 days.

EASA and FAA have made 25 hours recording time mandatory for aircraft with a mass 27,000 kg and more.

Ejectable recorders are not yet mandatory (actually FAA deems them not necessary) but are used on a few Airbus aircraft.

The goals are:

To ease investigation by providing a longer recording time to allow investigate cases like MH370. If the CVR had been recovered, two hours of recording would not have been enough.

To ease detection and recovery of the aircraft and its recorders, to prevent long search times with costly means as for AF447. Currently recorders are associated with an ultrasound beacon to find them under water, but the beacon stops transmitting when the battery is exhausted. Search and rescue (SAR) teams have then to rely on sophisticated rovers mapping the ocean floor rather than on simple acoustic or radio direction finding devices and sonars.

As regards for the possible loss of data, the past accidents have shown recorders can be successfully recovered and read with a high rate. Adding a recorder can help increase this rate.
The ejectable recorder is aimed to equip aircraft used on long range flights, with over-water portions out of radar sight. Aircraft for short range flights are easier to localize as they are always under radar surveillance, both secondary radars relying on the transponder and independent primary radars used by military to detect non-cooperative intruders close to or in their sovereign airspace.
Automatic Deployable Flight recorders (ADFR) separating from the aircraft in case of crash are used on Airbus (A350) and a few other Airbus models since 2019. They are located in the vertical stabilizer. The mandatory recorder (in this case one combining voice and data) is still present in the forward section of the aircraft. This fixed recorder is able to record data until the the sensors or the cables are destructed.

(Source: Leonardo)
ADFR principle, from SKYbrary:

Sensors detect the start of a crash.
Deployable unit releases from the aircraft. If the aircraft is in the air, aircraft movement assists the separation of the device. If the aircraft is underwater, the unit surfaces due to being lighter than water.
Deployable unit lands safely away from the crash site. If it lands on water, it can float indefinitely.
The ELT transmits the location and ID of the aircraft emergency beacon via satellite to SAR authorities.

The big improvement is the recorder is equipped with a buoyant radio beacon (ELT). The radio beacon can transmit its position to SAR satellites (Cospas-Sarsat); even if it drifts away from an ocean crash site, the first transmission will indicate the GNSS aircraft location.
Deployable recorders are a partial solution along with other surveillance projects like Space-Based ADS-B deployed by Aireon. Space-Based ADS-B allows ADS-B Out location messages ("extended squitters") sent by aircraft to be received from everywhere, including when flying over oceans. The place an aircraft crashed can be determined by Space-Based ADS-B if the aircraft is using this reporting mode.
There are also projects to stream recorder data via satellite and also to access them remotely. For instance EU is funding projects:

Progress on research into both the real-time tracking of aircraft and the possibility of accessing flight-recorder information without the flight recorder being physically present should be encouraged to improve the tools available to investigators for determining the causes of accidents and to enhance capabilities for preventing recurrent incidents.

An idea is to detect when this streaming must be triggered, rather than continuously streaming with a large throughput. Simulations conducted by the French BEA have shown this is feasible using existing satcoms.
Military aviation
Military aircraft have been fitted with ejectable recorders since a long time.

One change they should make is The pilot should not have the ability to wipe recordings. If they move to a SOLID state recorder then can also record nearly unlimited voice and frankly if the pilots or maintenance is doing something wrong they should not have the ability to cover that up. I have worked in environments where 100% of the time. You have zero expectation of privacy while on the job site. Aircraft cockpit environments should be held to similar standards with the CVR on all the time and record a month minimum. You can still have conversations, just know they are recorded.

@RowanHawkins: All modern recorders, voice or data or combined are digital and use memories. Whether pilots should accept new rules for the CVR is not exactly the topic (but I agree). Anyway, all these considerations will be outdated soon as the future of recorders is to use up-streaming to tracking satellites. Airbus has already solutions improving ACARS.

Yes. Self-ejecting flight recorders can be found on some new airliners, called Automatic deployable flight recorders.
They are currently available for the A320, A330, and a factory option on the A350. Reports say the A350 involved in the runway collision was equipped with one, though I could not find reports on whether it deployed and where it was found.
An alternate solution has been proposed to be a system which can uplink large amounts of data to a satellite network immediately before a crash. Ultimately, this may be the more reliable data recovery solution, but we aren't there yet in terms of satellite networks.

I'm pretty sure Starlink could do it today, but there are still a couple of problems: 1) Obiovusly, Starlink can only legally operate where they have a license, when they overfly a country they don't have a license for, they need to turn the radios off – that is the case currently in Greenland, India, Kazakhstan, most of Africa, China, and the "usual suspects" (Iran, Afghanistan, Russia, Belarus, Cuba), among others. 2) Vendor lock-in: there is no competitor. 3) SpaceX's rapid iterative development does not play nice with traditional certification processes.

No, Mainly because the devices need to record up to the end of the flight. It hasn't been until the last decade or so where our infrastructure could even manage the data flow that could allow external recording of that data to be streamed out of the aircraft, and in several of your examples happened where that infrastructure was not even available.
The other logical problem would be because the FDR and CVR are located in a specific area of the craft, it limits the area you would need to search for it greatly increasing the likely hood that it WILL be located. If you started popping it off it could be anywhere. Looking at "needs survive some force," build to the protection of the craft in question, I would much rather be in a crashing plane than having a crashing plane land on me.
Then there is the water scenario, the pinger only runs for a day or two if you are looking for a shoe box in the bottom mud of the ocean, it gets back to the locatability of the devices, if the pinger gets me to the aircraft, I can take longer than 2 days to recover it because I know where in the craft the device is located.
If it is just the boxes, it is very dark at the bottom of the ocean, an aircraft is pretty big, but not in ocean sized scales. Have someone else hide your wallet in a pitch black room with other stuff in it. Your search tool is a penlight with a weak battery you can only use from the floor above. Sadly this search is probably easier than the search for the Malaysian flight. That is because items need a few months under deep water to be really visible to sonar.

Actually, they can be very feasible. The recorder would deploy when certain G-force threshold is exceeded so staying with the airframe until crash. As for ditching, a deployable recorder is floating. The idea is that they would not sink with the hull but would eject and float at the time of water impact.

According to user71659's answer, Airbus already offers these for some aircraft. The "need to record up to the end of the flight" is handled by the deployable FDR (ADFR) being a backup unit, so the "old-style" FDR (or rather, the mixed CVDR) stays with the plane until the bitter end - although I'd imagine that the most valuable data is the one recorded before the "significant structural deformation or impact with water", not after.

All of the other answers above were created after mine. I was't aware of the Airbus Option, but I know Boeing doesn't have that capability. Like I say in my answer it hasn't been until recently that data bandwidth availability such as Starlink has been available. Ejectable FDR/CVR isn't something that can be retrofit into the thousands of planes already in the air ..easily.


text:
Are all the engines on a multi-engine plane the same?
In general, are there any differences between the engines on a multi-engine aircraft? Are engines designed to be specifically for the left or right wing, or inboard or outboard for a 4-engine aircraft?
What about the tail engine on a tri-jet like the DC-10? Is it the same as the two wing-mounted engines?

I don't know about jets, I'm guessing they're usually pretty much the same. There are some props with counter-rotating engines (to avoid having a critical engine) though which are obviously somewhat different. I don't know if they're mirrored completely or how it's done.

One classic example of two different engines was the Rutan Voyager: http://en.wikipedia.org/wiki/Rutan_Voyager

For modern jet engines, the engines are identical , but the engine mounts to the aircraft may be different. The most obvious difference would be "mirror image" mounts for left and right tail-mounted engines, or for the center engine of a 3-engine poane. With 4-engines, the inner and outer engine mounts may be different. For flight testing a new engine design, sometimes a mix of "new" and "old" engines of completely different types are used to minimize risk and provide reliable backup for the engine being tested.

I'd also mention the A380 where only the inner engines are equipped with reverse thrust. Though the engines themselves may be the same, this is a significant difference, at least more than mounts.

The Rutan Boomerang, https://en.wikipedia.org/wiki/Rutan_Boomerang, also has two different engines. From what I heard, it wasn’t for any particular design considerations, he just happened to have two engines that he wasn’t using.

Generally yes, but not always. If you include the APU as an engine, the answer would need to be different, but I understand your question concerns just the engines used for propulsion.
For jets, the direction of rotation doesn't make much of a difference, so the same engines can be used on all stations. However, with propellers the swirl does influence flying characteristics, especially at low speed, so special left- and right turning versions of turboprop and piston engines are available.
Maintenance and logistics become much simpler if only one type of engine is used. In the past, not so much emphasis was placed on this, so some airplanes used different types of engines. Examples were:

Convair B-36B, which combined piston and jet engines in the same airframe
Junkers G-38, which used a Junkers L88 inboard and a Junkers L8 outboard. Later, the outboard engines were also changed to the L88. This can be seen from the number of propeller blades: If the outer engines drive a two-bladed propeller, it is the earlier version with the L8.
Hawker-Siddeley Trident 3, which was almost a four-engined jet, because it had a small, tail-mounted RB-162 engine in addition to the three tail-mounted RB-163 Spey to provide thrust on takeoff.
Rutan Voyager, which used a bigger front and a smaller rear engine.

One difference may be 3-engine jets: the middle engine is mounted on the vertical stabilizer and the fuel exhaust tube is somewhat longer and slightly curved. It's just a question whether you consider this as a "different engine", but I think that if you count clockwise/anti-clockwise, you should also count this.

@yo': Normally the tail engine is the same and only enclosed in different cownling with special intake duct (intake has cold air, so it's easier to change then exhaust, which has hot gasses).

@yo': As for clockwise/counter-clockwise, there are two options. The engine can be reversed, in which case basically all parts are mirrored, which makes the engine very different, or there can just be a different gearbox, in which case the engine itself is the same. Both options are used somewhere.

@JanHudec An example of right/left handed engines is the P-38: "Counter-rotation was achieved by the use of "handed" engines, which meant that the crankshaft of each engine turned in the opposite direction of its counterpart."  From http://en.m.wikipedia.org/wiki/Lockheed_P-38_Lightning

About the Trident, are you referring to the "booster" engine on the Trident 3? That was not an APU, and I can't find anything about the APU providing thrust.

@yo' Not all 3 engine jets have the engine mounted on the vertical stabilizer.  The 727  intake is much higher than the actual engine, which is mounted at the same height as the other two engines.

@fooot: Thanks for the hint. Seems I got confused, the booster on the Trident 3 was where the APU was on the Trident 2. But it would have made sense to couple a generator to it.

As this image shows the Navy flew a plane with two propeller engines and two jet engines.  Read more about it here.

From wikipedia:

Beginning with the P2V-5F model, the Neptune became one of the first
  aircraft in operational service to be fitted with both piston and jet
  engines. The Convair B-36, several Boeing C-97 Stratofreighter,
  Fairchild C-123 Provider, and Avro Shackleton aircraft were also so
  equipped. To save the weight and complexity of two separate fuel
  systems, the jet engines on the P2Vs did not burn jet fuel- they
  burned the same fuel as the piston engines: 115–145 Avgas. The jet
  pods were fitted with intake doors that were kept closed when the
  J-34s were not running to prevent them from windmilling, allowing for
  economical piston-engine-only long-endurance search and patrol
  operations. In normal U.S. Navy operations, the jet engines were run
  at full power (97%) to expedite and assure all takeoffs, then shut
  down when the aircraft reached a safe altitude. Also, the jets were
  started and kept running at flight idle during low-altitude (500 feet
  during the day and 1,000 feet at night) anti-submarine and/or
  anti-shipping operations at sea as a safety measure in case one of the
  radials developed problems.

The picture is good but this is otherwise pretty light on details. Adding more would help improve this.

We encourage users to post at least a basic amount of information, because if the link dies in the future, the post becomes almost useless. Providing more than just a link also makes it more worthy of upvotes.

Ah.  Ok, then I'll update with more info.

The place to find the true answer is in the Type Certificate Data Sheet. This document will tell you exactly what engines can be installed on an aircraft.
For example on the DC-10-10:

3 General Electric CF6-6D, CF6-6D1, CF6-6D1A, CF6-6K or CF6-6K 2 Turbofan Engines. (CF6-6D and CF6-6K engines may be intermixed in accordance with Appendix XXII of the applicable FAA Approved Airplane Flight Manual. CF6-6D1 and CF6-6D1A engines may be intermixed in accordance with page 2.1 of Section IVB of applicable FAA Approved Airplane Flight Manual.)

Usually the Engines on the same plane are of the same model (or type). As mentioned in an answer earlier the rotation direction of jet turbines doesn't affect handling, however, the engines installed on either sides must have same thrust as to make the handling easier.
One example for differing engines however is A380, where the inboard engines are equipped with thrust reverser and outboard engines don't have them. There are several reasons for doing this, not having thrust reverser makes engine comparatively simpler to build and maintain.

The "Super 27" replaced the outer engines of the Boeing 727.
The problem with re-engining a trijet is the necessity of re-engineering the center engine's casing and intake, wspecially if an S-duct is involved.  That is so complicated that re-engine plans typically left that engine alone.
So it was with the Super 27 with a swap to the more powerful JT8D-217. I have jeard of schemes to put even more modern engines on the outer pylons, but cannot find the link.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Will I fail a skills test for doing a constant-aspect climb-out?
The climb-out for one of the runways at my local aerodrome is over a residential area, with fields to the left. My instructor has had a bee in his bonnet lately about engine failure after take-off if the take-off run is longer than it should be (e.g. after a touch-and-go). He's started encouraging all of us to perform constant-aspect climb-out into the circuit, where we start the turn onto the crosswind leg at the aerodrome boundary regardless of height, and keep the runway end at the same declination in our field of view as the turn progresses. This means we point towards the fields much earlier on, but it hurts the climb performance to do it.
Although I can see the wisdom of being ready for EFATO, I'm concerned about practising a non-standard circuit which contravenes the Noise Abatement Procedure. (The NAP forbids turns below 500 ft, although it is overridden by measures to ensure flight safety.) When I come to do my PPL skills test, would this unusual circuit be acceptable, or will I have to fly a standard circuit with a straight climb-out, turning onto base at 500 ft? I'm in the UK and studying towards an EASA PPL(A).

Sounds like a good pre-flight discussion to have with your examiner.

@RonBeyer This is a great comment.  I've seen people act as if it's not the done thing to discuss the flight with the examiner prior to walking but I believe the correct position is that to discuss all flights first, especially when someone on board can act as a safety pilot, is very good practice.

@RonBeyer That's a good suggestion, but it might be a bit too late if I've been learning the wrong thing all along.

@DanHulme You've flown from more than one airport using standard procedures right? It would be a problem if you couldn't abide by a standard departure pattern because you weren't taught it, but I'm guessing you should have been using a standard procedure at other airports right?

@Simon Exactly, discussing your flight, the examiners preferences, procedures, etc is a good thing to get out of the way on the ground where you aren't distracted by not bending up an airplane or under pressure of a testing environment. Talking it out with the DPE only shows that you want to be prepared and is a good thing, its not weakness to discuss how they would like to see certain procedures performed.

I fly in the UK, and while there is certainly a "standard circuit" (ie pattern) in the syllabus there are many, many fields that have a non-standard circuit for one reason or another. This is often to avoid antagonizing unfriendly neighbors.
There's also the fact that the NAP is not a rule or regulation, it's more how the airfield management would like you to fly.
I personally don't like the constant aspect climb-out idea at all, it will mean you are paying more attention to aspect than airspeed and looking out for other traffic. It also means that the pattern traffic is not predictable for other visiting your airfield or by others in the pattern. So I would push back on your instructor on this.
As for whether you will fail your skills test if you do it would depend on the examiner's view, and what they are looking for. While a constant aspect departure shows skill, I personally would take a dim view of people doing it despite the published departure procedure, even if it is non-binding. I would discuss it with your examiner ahead of time, and practice your circuits how they say they want it, or at least discuss it with them on the day.

I agree. Discussing it with the examiner is a great idea. However, bottom line: it's a great tool to have in your bag of tricks when you need to be able to do it. It doesn't hurt to practice it. Knowing when it's smart to use the constant aspect climb and when it's not is an even better tool. Also, I don't know about the UK, but there are US airports that take a dim view of NAP violators. They like to send threatening letters and offer to ban you from operating at their fields if you don't follow the NAPs. My philosophy is to do what is safest.


text:
What special handling characteristics or techniques do biplanes have?
How does flying a biplane differ from flying a monoplane? Apart from the increased drag and lift, how does it handle differently, and what effect does that have on manoeuvring?
In particular, I'm interested in differences between a biplane and a hypothetical equivalent monoplane, not differences between vintage airplanes and modern ones, or differences between aerobatic and utility airplanes.

There are a lot of opinion-based answers and answers copied from other sources, but very little actual evidence. In some ways, this question is meaningless, unless someone can come up with two actual airplanes that have the same engine and airframe, but with different wings.

I thought the design choice between a biplane and single wing was a matter of wing area, and early plane designers used box designs because of the weight advantage over a long, heavy strong spar.

@rbp The answers tend to focus on design decisions, but I was thinking more from a pilot's point of view. When I fly my biplane, what am I doing differently from just a slow, old monoplane?

having flown both, the real answer is not much. you'll find a bigger difference between the gear styles, tailwheel and tricycle, than between a single wing and a biplane.

Better visibility with a monoplane.

What is the stall speed of the Tiger Moth?

Short answer:
For clarification: Equivalence between biplane and monoplane means than both have the same wing area and the same engine. Then the main differences in maneuvering are:

The biplane has better roll acceleration than an equivalent monoplane.
The biplane has a higher roll rate than an equivalent monoplane at the same speeds.
All biplane flying takes place at lower speeds, resulting in a lower space requirement for all maneuvers. This also means that inertial effects are less pronounced: When pulling up, there is less kinetic energy available for climbing, so (for example) hammerhead turns will end with less altitude gain.

Differences in handling: The biplane has

lower aileron forces for the same roll rate at the same speed
lighter control forces overall due to lower flight speed

Differences in performance:

shorter take-off and landing distances
a lower stall speed
a much lower maximum speed
a lower optimum cruise speed and range
a lower power requirement due to the lower flying speeds, or if both use the same engine, a better power-to-weight ratio

when compared to an equivalent monoplane. These differences are most pronounced if the airplane carries just the pilot and not much payload.
Flying techniques are the same as for monoplanes. Indirectly, differences are likely due to differences in design. Example: Few biplanes profit from having a retractable landing gear while gear retraction makes sense for monoplanes with higher power loading (installed power relative to wing area).
Explanation
Biplanes have two major differences:

Smaller wing span at the same wing area, and
Wire bracing results in very lightweight biplane wings.

The smaller span reduces roll damping and roll inertia, so a biplane will accelerate into a roll more quickly than an equivalent monoplane and will reach a higher roll rate. This is the main difference in maneuvering.
The smaller wing span results in more induced drag if both have the same mass and the same speed. With wire bracing, this condition is unrealistic, and an equivalent biplane will be much lighter. If the structure is a substantial part of the aircraft's mass (this is typical for aerobatic airplanes), the result can easily be less induced drag, despite the lower span, and also lower wing loading. This in turn means that both fly at different speeds: The biplane will be able to fly much slower, but the aerodynamic drag of the bracing will restrict it to low speeds. This also means inertial effects are less pronounced: The lower mass and lower speed of the biplane combine for a marked difference to the equivalent monoplane.
For aerobatic displays this is ideal: All action takes place close to the audience, and the biplane will need a much smaller area for all maneuvers than an equivalent, but heavier monoplane. The downside are low maximum speed and low range.
Another difference in performance are much shorter take-off and landing distances due to the lower wing loading, which results in a lower stall speed. The optimum endurance and optimum range speeds are lower than those of an equivalent monoplane as well, so all biplane flying happens at lower speeds, which is beneficial for training aircraft.
Since control forces are proportional to dynamic pressure, a biplane will have lower control forces than an equivalent monoplane. Here equivalence also means that the relative chord of all control surfaces is the same. In reality, a good designer will select a higher relative chord for the biplane's control surfaces to ensure that control forces are above their required minimum.
The heavy, unreliable engines of the early years made biplanes the ideal way of taking to the air. Once engines became more powerful and allowed higher payloads, the monoplane became better suited to carrying passengers and freight at higher speeds and over longer distances.

how much of these advantages can be attributed to the lighter weight of biplanes versus single-wing airplanes, for the same wing area? aside from the box kite construction, most biplanes have fabric wings, versus metal skin and wood or metal spars of single-wing planes, making them much lighter.

@rbp: Equivalence means that both should use the same materials and processes. With the lower dynamic pressure of biplanes, they have less problem with flutter and gust loads, so it is possible to build them of wood and fabric. Now both are not equivalent anymore, like with the gear retractability I used as an example. Optimizing each will mean to lose the basis for a fair comparison. When optimized designs are compared, the biplane will leverage the lightweight wing design to also save weight in all other parts. In the end, all this is really a consequence of the box kite construction.

"the lower dynamic pressure of biplanes, they have less problem with flutter and gust loads" can you improve the answer to cite a reference? or maybe explain better why?

@rbp: Both speed and dynamic pressure are factors in flutter. The speed determines the frequency of aerodynamic oscillations, and the dynamic pressure determines how much energy is involved. Oscillations must not have similar frequencies than structural eigenmodes to avoid flutter, and a lower speed means less structural stiffness is needed to ensure this. Gust loads are again proportional to both, and higher flight speed produces steeper gradients of force changes, and the force change itself is proportional to dynamic pressure.

Why has a biplane a lower stall speed than a monoplane with equal wing area?

@Koyovis: Interference between both wings. The high pressure below the upper wing is reduced by the suction on the lower wing. A clean single wing can produce more lift.

@PeterKämpf The top wing still stalls though, doesn't it?

@Koyovis: Given enough angle of attack, both will stall. Staggered wings make the top wing behave a little bit like a slat and delays stall on the bottom wing, but only because the bottom wing sees a reduced angle of attack.

If a monoplane used kingpost and wire bracing to mitigate its structural weakness, how much does that help it to catch up with the box kite? (assuming otherwise similar lightweight construction, engine power). Obviously both have wire drag in that case...

@BrianDrummond: This helps a lot and makes the single wing structurally very similar to one biplane wing.

@Koyovis Reading your question and my answer now makes me realize that I gave you a poor answer. The lower stall speed of the biplane is entirely due to its much lighter wing and lower maximum speed which in turn allows also the fuselage to be built lighter. So the biplane, despite its wings producing less lift per area than the monoplane, will stall at a lower speed due to its much lower wing loading.

In a nutshell, the advantages and disadvantages are:

Advantages

Biplanes (or triplanes) can usually lift up to 20% more than can a similarly sized monoplane of similar wingspan. A biplane will
  therefore typically have a shorter wingspan than the equivalent
  monoplane, which tends to afford greater maneuverability.
The struts and wire bracing of a typical biplane form a box girder. Particularly when divided into bays, this permits a very light but strong and rigid wing structure. This allows a biplane to fly with very little power...

Disadvantages

Each wing negatively interferes with the aerodynamics of the other, requiring greater overall surface area to produce the same lift as the
  equivalent monoplane.
A biplane typically also produces more drag than a monoplane, especially as speed increases.

Technological Advancements
In early days (1900-1930s), the biggest advantage of biplanes was having twice the surface area and a rigid structure to support the wings. But at present, high strength carbon fiber reinforced plastics made it possible to build very high aspect ratio wings without any (or little) external support. With the advent of steel, then aluminum air frames, the previous considerations were mute, and monoplanes have become more common than biplanes.

NASA has a historical discussion about these.

What do you mean by the second advantage point? How does the box girder structure affect power (beyond increasing it because it has higher drag)?

@JanHudec I didn't paste the entire explanation from Wikipedia but have done it now. If you think it still doesn't fully clarifies the point, please let me know and I'll update it.

It doesn't. Because it doesn't make sense on Wikipedia either. The box girder allows building it from weaker material. But it has larger drag, and therefore requires more engine power, at all speeds.

@JanHudec: The statement about power is correct, but only because the biplane can fly much more slowly. At the same speed it needs more power.

@PeterKämpf : indeed, so slowly that an Antonov 2 reportedly doesn't have an official stall speed. If you fly slow enough, you'll just descend with it like with a parachute.

@vsz … only when the c.g. is all the way aft. Biplanes still obey the laws of physics, and this mushing stall is only possible close to neutral stability.

@PeterKämpf: Biplane may be able to fly more slowly, but that does not follow from the use of box girder structure. And with worse L/D it's not clear it will need less power even at lower speed.

This is a good discussion of the advantages and disadvantages of each, but I was asking specifically about the differences in flying from the pilot's point-of-view.

Note that adding more planes will not always increase the mentioned advantages: http://images.slideplayer.de/1/666186/slides/slide_19.jpg

@JanHudec right, in fact, the L/D curve tells us that you need MORE power to fly slower, not less.

@JanHudec: Mostly wrong. Don't use slightly nonsensical designs like the Beech Staggerwing or DH89 Dragon Rapide for a comparison. A well designed biplane will have low wing loadings which no equivalent monoplane can match. Remember that the power requirement goes up with the cube of airspeed. A little less speed goes a long way in reducing power needs.

@PeterKämpf: Cube of airspeed? Power = thrust * speed. And thrust only increases with square of speed above Vy. Biplane will have lower Vy (due to lower wing loading), but need higher thrust at it's respective Vy due to lower L/D. The two differences work against each other.

@JanHudec: Yes, and then drag ~ speed². Makes power ~ speed³ when thrust = drag (as it should be). A prop will give more thrust at lower speed, the airplane is slower AND lighter - I don't see how it would need MORE power. This discussion should not hide in the comments, but might spawn a good question in itself.

@PeterKämpf: And what about induced drag?? That one is inversely proportional to speed, isn't it? And most aircraft don't cruise all that faster than Vy, the speed of minimum drag.

@JanHudec: Induced drag is also proportional to lift, and a lighter airplane requires less of it. Again, a small speed difference will already make a big difference in power.

You may wish to mention visibility.  An extra wing makes it a little bit harder to scan for traffic.

I love this conversation -- I've learned a lot. But it seems to me that from a handling standpoint, it would be difficult to impossible for a blindfolded pilot (ok, one wearing a "view limiting device") to tell the difference between the two. And is there more difference between flying a EX300 and a SIAI-Marchetti, than there is between an EX300 and a Pitts?

There is no principial difference in handling. Both have the same set of control surfaces and stability is achieved using the same methods, so the piloting techniques are the same. There will usually be difference in performance. Biplanes loose speed faster, but are slightly more manoeuvrable.
The main advantage of biplane is that it can be built with less strong materials, because the wings are shorter and the box girder bracing structure distributes the loads very well. This often meant lower weight which offset the increased drag.
The main disadvantage of biplane is that shorter wingspan means lower aspect ratio and therefore higher induced drag, which dominates at low speed, and the larger frontal area and surface area (interference between the wings means they are less efficient than they would be independently) mean higher form drag, which dominates at high speed. So a biplane needs a stronger engine for the same weight and will not glide as far should the engine fail.
Before cantilever (without bracing) wings were developed, the bracing didn't allow monoplanes to achieve that much lower drag and their higher weight cancelled the little aerodynamic advantage they had, so biplanes dominated. With cantilever wings the aerodynamic advantage of monoplanes became more important and biplanes almost disappeared.
A few aerobatic biplanes (like the famous Pitts SC1) remained, most likely because with shorter wingspan they have lower moment of inertia in roll and therefore roll slightly more easily.

Biplanes handle like bricks. They lose altitude more quickly at low speed, and they are much easier to stall. They have more drag and will enter a spin more readily. They are much more susceptible to cross winds than monoplanes and therefore can only take off and land in relatively moderate conditions, since they will flip over easily.
The advantage they have is that they can turn much more quickly than a monoplane, so they are nowadays mainly used for acrobatics, like Pitts Specials.
During the war pilots flying Fairey Swordfishes and Fireflies were in great danger from the much faster Bf109s. They would escape by plummeting to the ocean in a virtual free fall then pulling out at the last moment. Any Messerschmidt pilot dumb enough to follow them in this maneover would be flying their last mission.

Biplanes have typically higher wing areas for the same sized aircraft. I remember I biplane that I took a ride in crashed not on hour later because on short final he caught a wind gust and ended up upside down on the runway!
(Everyone was okay, but you never forget the tin can hanging off a bike sound of an airplane crash.)

During WWI, the Fokker Triplane was really maneuverable. http://acepilots.com/wwi/fokker_triplane.html "Undoubtedly the most famous fighter of World War 1, the Fokker Dr I was a revelation when it entered service on the western front in 1917. Manfred von Richthofen’s JG 1 was the first Jasta to completely re-equip with the new fighter, and in the skilled hands of its numerous aces the Dr I proved a formidable opponent. The Dr I remained in service on the Western Front until replaced by the superior Fokker D VII in May 1918.  ...

Just weeks prior to that, however, Germany’s leading ace, the great ‘Red Baron’, had been killed at the controls of a Dr I.
The revolutionary triplane design adopted by Fokker was inspired by the equally successful Sopwith’s Triplane, and although built in remarkably small numbers, the Fokker Triplane legend has made it the best known aircraft to emerge from World War 1."

Ooh, and the British had a Triplane too. Also very effective it seems, altho not as well known as the Fokker Triplane http://thevintageaviator.co.nz/projects/sopwith-triplane/history-sopwith-triplane  Picture http://thevintageaviator.co.nz/sites/default/files/styles/media_gallery_thumbnail/public/images/sopwith-triplane-replica/soptripe_024_fly.jpg?itok=jtvmdyD6

Fokker Triplane picture https://www.plane-encyclopedia.com/wp-content/uploads/2016/06/Dr1-Dark-Red-900x597.jpg  Arrgh, link won't work ... But if pasted into a browser it will ...


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How prevalent are anti-explosion bladders in fuel tanks?
Some aircraft fuel tanks are fitted or retro-fitted with internal bladders, to prevent fuel evaporating inside the tank. I've heard of this being a requirement for at least one type, perhaps because of a poor fuel tank design, but how commonplace is it? How many aircraft have these bladders?

In Australia there were a string of fatal accidents involving Robinson R44 helicopters. The crashes were survivable but the old rigid fuel tanks burst and the fire caused the fatalities. As a result the authorities mandated that all R44s be retrofitted with the new bladder-like fuel tanks. http://atsb.gov.au/newsroom/news-items/2015/atsb-calls-on-us-and-european-regulators-to-act-on-reducing-helicopter-fires.aspx

According to this wikipedia article :

Most jet fighters and all US military rotary wing aircraft have some type of self-sealing tanks. Military rotary wing fuel tanks have the
additional feature of being crashworthy. High altitudes require the
tanks to be pressurized, making self-sealing difficult. Newer
technologies have brought advances like inert foam-filled tanks to
prevent detonation. This foam is an open cell foam that effectively
divides the gas space above the remaining fuel into thousands of small
spaces, none of which contain sufficient vapour to support combustion.
This foam also serves to reduce fuel slosh. Major manufacturers of
this technology include Hutchinson, Amfuel (Zodiac) (formerly
Firestone), Meggitt (formerly Goodyear), Robertson Fuel Systems, GKN
USA and FPT Industries. FPT is now part of GKN. For military use,
tanks are qualified to MIL-DTL-27422 (includes crashworthiness
requirements) or MIL-DTL-5578 (non-crashworthy). An aircraft fuel tank
sometimes consists of several interconnected fuel cells. The
interconnecting hoses are typically also self-sealing.
In addition to fighter aircraft some military patrol vehicles and
armoured VIP limousines feature self-sealing fuel tanks.
Self-sealing fuel tanks using military technology are also required in some motorsport categories.

Given the fact anti explosion tanks were invented in 1917, and are in use since then, and have undergone many technological advances since then, they are being used quite commonly and in most of the planes where pressurization is required.